Disk drives as data storage devices typically comprise a stack of rotatable disks to which data is written and read by way of magnetic heads that are movably supported with respect to surfaces of the disks by a like number of head suspension assemblies. One such head suspension assembly is typically movably supported relative to each disk surface so that a magnetic head can be selectively positioned relative to a data track of the disk surface, as such magnetic head is provided on an aerodynamically designed slider so as to fly closely above the disk surface while the disk is spinning. Each head suspension assembly is normally connected to an actuator arm for moving the head suspension and head over the disk surface for data writing and reading, and each actuator arm is connected to be driven by a voice coil drive device. Such an assembly allows each magnetic head to be independently controlled for positioning relative to specific data tracks of the disk surfaces.
The density of data tracks on such disk surfaces has been increasing in order to obtain greater data storage within a given disk surface area. Specifically, the data tracks themselves have become narrower and the radial spacing between tracks has decreased in order to increase disk data density.
In order to provide a second level of adjustability to a magnetic head as provided to a slider of a head suspension assembly and to obtain greater data resolution, microactuators have been developed. In general, a voice coil drive provides a first adjustability for course positioning of the magnetic head and a microactuator can then provide a fine adjustability for resolution of data tracks within high density disk drives. Such systems are considered dual-stage actuated suspension systems.
A common microactuator comprises one or more elements of piezoelectric crystal material such as lead zirconate titanate (PZT) as such elements are strategically provided at one or more locations along a head suspension assembly. A head suspension assembly typically comprises a base plate for connection with an actuator arm, a load beam including a base plate portion, a spring portion and a rigid region, and a flexure for supporting a slider with a magnetic read/write head. The interconnection of the flexure and load beam allow for pitch and roll movement of the aerodynamically designed slider relative to a spinning disk surface as the slider flies on an air bearing created by the spinning disk.
Microactuators have been developed to work on base plates, load beams and flexures by causing a distortion of material, typically stainless steel, by providing an electrical field across fixed elements of piezoelectric material. The controlled application of a voltage difference across a piezoelectric microactuator, such a PZT, causes the piezoelectric microactuator to expand or contract, in order to distort the base plate, load beam or flexure and thus controllably provide a fine movement of the slider and head with respect to a specific data track. Microactuators are sometimes provided in pairs for controlled deflections acting together by applying similar or opposite polarity electrical fields to the piezoelectric element pairs depending upon the location and arrangement of the piezoelectric element pairs.
In order to controllably actuate such piezoelectric microactuators, positive and negative electrical connections are provided to each piezoelectric microactuator. Conductors as are typically provided along head suspension assemblies extending to the head for read/write functionality and for providing voltage across the piezoelectric microactuators of a dual stage actuated head suspension. Such conductors can be provided as wires or as traces of flex type circuits that can be formed integral with or attached along the load beam of a head suspension assembly. Utilizing conductive traces, connection pads are typically provided at the end of the conductive traces for connection with positive and negative voltage surfaces of each piezoelectric microactuator.
Conductive traces themselves are usually comprised as a laminate type structure including a stainless steel structural or support layer with an insulator layer between the stainless steel and any number of conductive traces as may be formed of any electrically conductive material, such a copper. The connection pads are made by creating a circular pad of the stainless steel, insulating material and the copper layer, followed by removing (such as by etching) an area of the stainless steel and insulating material from below the copper layer to provide access to the copper layer through the other layers of the connection pad. Such connection pads are also known to be provided as gold plated copper pads.
In order to electrically and physically connect the electrical connection pad to a surface of the piezoelectric microactuator, which surface also may be gold plated, conductive epoxy is known to be utilized. Conductive epoxy can comprise a conventional epoxy resin that is impregnated with silver flakes and/or particles of sufficient silver density within the resin to render the epoxy capable of providing an electrical connection through the epoxy resin. In addition to including sufficient silver particle density for electrical conductance though the epoxy connection, it is also been determined that good electrical connection between the gold plated copper pad surface and the silver particles of the epoxy is needed. Intermittent electrical connections can result from faulty or insufficient electrical connection at this electrical pad to epoxy interface.
Faulty electrical connection between the connection pad of the flexible circuit and the surface of the piezoelectric microactuator can result from separation of the silver particles at the interface of the conductive epoxy and the gold plated copper surface of the connection pad. This situation is referred to as a resin rich formation at the interface of the gold plated surface of the connection pad with the conductive epoxy. The formation of resin rich zones along the interface with the surface of the connection pad reduces conductivity from the connection pad to the conductive epoxy. Resin rich zones are believed to result sometimes during the curing process of the conductive epoxy due to material differences between the copper connection pad and the epoxy. For example, thermal expansion of one material, such as the copper, during curing can affect the interface as well as material shrinkage of a material, such as the epoxy, during its curing. Other factors can include physical conditions, such as vibrations or otherwise, as may affect this interface during the epoxy curing process.